Uhh, I don't know, but it is some kind of basic theme. When you install Windows Server with Desktop Experience but remove the Shell GUI (I don't know if it is the right name for it), it is the same as in the picture. Also if you install Windows Server without the Desktop Experience, then it has also the same as in the picture but without the Desktop.
Sorry, I can only help you if you find a file that ends in either .deskthemepack or themepack. or theme. Look in Windows / Resources / Themes
There is only a folder named "aero" with an "aerolite.msstyles" and the en-US file "aerolite.msstyles.mui". Even if I copy all files in the exact same folder to Windows 10, nothing visually changed.
I think if you want enable the aerolite.msstyles you need to duplicate the C:\Windows\Resources\Themes\aero.theme, call it aerolite.theme and then edit it in notepad for example and change aero.msstyles to aerolite.msstyles. oh you said not the aerolite theme. then i'm not sure what you mean
Actually I think it is the aerolite theme but I mean the window buttons on the picture, they are different. I think they are exclusive to the Server Core OS.
They are in .msstyles file. OP just need to install the server msstyles with a modified .theme and enable it from settings/control panel.